1. Technical Field
The invention relates to an endoscope objective lens and an endoscope, and more particularly, to an endoscope objective lens that is appropriate for use in such a configuration that a prism for deflecting a light path or the like is disposed between the endoscope objective lens and an image formation surface, and an endoscope including the endoscope objective lens.
2. Description of the Related Art
Generally, endoscopes are used for observing the inside of a patient's body, performing treatment for a patient, and the like. As the endoscope, widely used is a direct-view type endoscope that is used to observe in the longitudinal direction of an insertion section. In the direct-view type endoscope, an observation objective lens is disposed in a front end of the insertion section, which will be inserted into the inside of a human body, of the endoscope, and a solid-state imaging device is disposed on an image formation surface of the objective lens. In this type of the endoscopes, a light receiving surface of the solid-state imaging device is frequently disposed to be parallel to the longitudinal direction of the insertion section. Under such a configuration, generally, a light-path deflecting prism for deflecting a light path by 90 degrees is disposed between the objective lens and the solid-state imaging device.
In an optical system of the endoscope having the above configuration, it is required to have a long distance from a final surface of the objective lens in which the light-path deflecting prism is inserted to the image formation position, that is, a long back focal length. The applicant has described an endoscope objective lens having a long back focal length and having a four-group, six-element lens configuration in JP 2004-205779 A (corresponding to US 2004/0125469 A).
However, it is preferable that a filter such as a low-pass filter and/or an infrared cut filter is inserted between the final surface of the objective lens and the image location as well as the light-path deflecting prism that is minimum required. Also, in consideration of working accuracy, assembling accuracy, and the like of the light-path deflecting prism, the filter and the like, it is desired to have a longer back focal length.
On the other hand, in order to increase the depth of field, endoscope objective lens often include an optical system having a large F number, that is, a dark optical system. Accordingly, it is less likely that spherical aberration, coma aberration, and the like become significant factor to determine the image quality. Also, the lateral chromatic aberration would become a major factor to determine the image quality. In particular, in accordance with a trend of high density solid-state imaging devices, it is required to sufficiently correct the lateral chromatic aberration. As the lateral chromatic aberration increases, not only color blurring in a peripheral image occurs but also resolution in the peripheral portion decreases, which has influence on the observation or diagnosis for the inside of a lumen with the endoscope.
Accordingly, both a long back focal length and a good lateral chromatic aberration are required. In order to correct the lateral chromatic aberration, it is advantageous to dispose an optical member for correcting the lateral chromatic aberration in a position far apart from the aperture diaphragm. Also, as the optical member is disposed in a position closer to the image formation surface, a strong correction effect can be achieved. However, as the back focal length increases, the maximum height of light rays in this optical member decreases, and the effect of correction of the lateral chromatic aberration is weakened, thereby making it difficult to correct. In particular, in an optical system having the back focal length 2.5 or more times as long as the focal length, no lens is disposed in a position close to the image formation surface. Therefore, it is not easy to correct the lateral chromatic aberration.